The present invention generally relates to storage of information and more particularly to a magnetic storage device having a high-sensitivity magnetic sensor and a signal processing circuit for processing an output of the high-sensitivity magnetic sensor.
With increase of recording density in magnetic storage devices called hard disk drive, recent, leading-edge hard disk drives generally use a GMR (giant magneto-resistive) sensor in a magnetic head for picking up information from a magnetic disk. A GMR magnetic sensor is a device that changes a magneto-resistance thereof in response to an external magnetic field and is able to detect a feeble magnetic field produced by a tiny magnetization spot formed on a magnetic disk.
On the other hand, a GMR magnetic sensor has a drawback in that the magnetic polarization of the magnetic materials used therein easily undergoes reversal. Such a reversal may be caused by an electrostatic discharge and causes an inversion of polarity in the output signal representing the result of the reading operation. Further, such a reversal of the magnetic polarity may induce a distortion in the electric signal produced by the GMR magnetic sensor. When such an inversion of polarity or distortion is caused in the output signal, the desired reproduction of the information signal is not possible or severely impaired.
In view of the foregoing drawback of the GMR magnetic sensors, there is a proposal to use a spin-valve magnetic sensor in a magnetic head for reading the information from the magnetic disk.
FIG. 1 shows an example of a magnetic disk device 1 according to a related art.
Referring to FIG. 1, the magnetic disk drive 1 includes a magnetic disk 2 accommodated in an enclosure 10 having a cover 11 and stores information on the magnetic disk 2 in the form of concentric tracks. The magnetic disk 2 is mounted on a spindle motor 6 for rotation, and a floating magnetic head 5 scans over the surface of the magnetic disk 2. The magnetic head 5 is mounted at an end of a swing arm 7, wherein the arm 7 is connected to a voice coil motor 8 and the voice coil motor 8 actuates the arm 7 for swinging motion. With the swinging motion of the arm 7 thus caused by the voice coil motor 8, the magnetic head 5 scans over the surface of the magnetic disk 2 generally in a radial direction thereof. Thereby, the magnetic head 5 is controlled so as to trace a desired track on the disk 2.
The voice coil motor 8 is supplied with an electric signal from a read/write amplifier 9 for actuating the arm 7, while the read/write amplifier 9 further supplies an electric signal to the magnetic head 5 via the arm 7 for writing or reading of information on or from the magnetic disk 2. Thus, in response to the electric signal, the magnetic head 5 senses, or alternatively induces, a magnetization on the magnetic disk 2 and writing or reading of information is achieved on or from the magnetic disk 2.
It should be noted that the electric signal thus supplied to the magnetic head 5 from the read/write amplifier 9 corresponds to the data created and supplied from a host device (not shown), wherein the host device supplies the data to a circuit substrate 4 of the magnetic disk device 1 via a connector 3, and the electric circuit provided on the circuit substrate 4 converts the data to the electric signal.
In the construction of FIG. 1, it should be noted that the magnetic disk 2, the magnetic head 5, the spindle motor 6, the arm 7, the voice coil motor 8 and the read/write amplifier 9 are accommodated in the enclosure 10 having the cover 11.
FIG. 2 shows the construction of the signal processing system used in the magnetic disk device 1 of FIG. 1 in the form of a block diagram.
Referring to FIG. 2, the processing system is formed on the circuit substrate 4 and includes an HDIC (head IC) unit 13 that amplifies an output signal produced by a GMR magnetic sensor or spin-valve magnetic sensor of the magnetic head 5. The HDIC unit 13 is also called a head amplifier and is provided on the magnetic head 5 together with the GMR or spin-valve magnetic sensor. The output signal of the magnetic sensor 5 is then supplied to an RDC (read channel) unit 14 on the circuit substrate 4 wherein, the RDC unit 14 demodulates the original information recovered from the magnetic disk 2 by the magnetic sensor 5 in an encoded form, by conducting a sampling process.
The information thus demodulated is then supplied to an HDC (hard disk controller) unit 15 on the circuit substrate 4, wherein the HDC unit 15 transmits the information thus demodulated by the RDC unit 14 to a host device via the connector 3 not shown in FIG. 2. Further, the HDC unit 15 recovers the information related to servo control of the magnetic disk 2 or tracking control of the magnetic head 5 from the output of the RDC unit 14 and supplies the same to an MCU (micro-control unit) 16 provided also on the circuit substrate 4. Thereby, the MCU 16 controls the spindle motor 6 and the voice coil motor 8 via a servo control circuit 17 such that the magnetic disk 2 rotates at a predetermined, controlled speed as represented in FIG. 2 by an arrow A.
The MCU 16 further controls the operation of the RDC unit 14. Under control of the MCU 16, the arm 7 is caused to swing as represented in FIG. 2 by an arrow B, and the magnetic head 5 traces a track formed on the magnetic disk 2.
FIG. 3 shows the construction of a spin-valve magnetic sensor 22 provided in the magnetic head 5 for reading the magnetic information from the magnetic disk 2.
Referring to FIG. 3, the spin-valve magnetic sensor 22 is constructed on a substrate 23 of a magnetic material constituting a yoke and includes a free layer 18 typically formed of a ferromagnetic material such as a Ni-Fe alloy, a non-magnetic layer 19 formed on the free layer 18 of a non-magnetic material such as Cu, a pinned layer 20 of a ferromagnetic material such as a Ni-Fe alloy, and a pinning layer 21 of an anti-ferromagnetic material such as an Fe-Mn ordered alloy.
The pinning layer 21 is magnetized in the direction as indicated in FIG. 3 by an arrow P and creates a stable magnetic field associated with the magnetization P, wherein, due to the anti-ferromagnetic nature of the pinning layer 21, the magnetization P does not change easily even when the external magnetic field is changed. Due to the stable magnetic field thus created by the pinning layer 21, the magnetization of the pinned layer 20 is fixed on pinned in the counter direction as represented in FIG. 3 by an arrow Q. On the other hand, the magnetization in the free layer 18, which is separated from the pinned layer 20 by the non-magnetic layer 19, changes the magnetization thereof in response to the external magnetic field created by the magnetization spot formed on the magnetic disk 2, as represented by arrows S and T.
FIG. 4 shows the operation of the spin-valve magnetic sensor 22.
Referring to FIG. 4, it can be seen that the pinned layer 20 is magnetized in the fixed direction represented by the arrow Q, while the magnetization of the free layer changes or rotates in response to the magnetic signal on the magnetic disk 2 as represented by the arrows S and T. Thereby, the angle between the magnetization of the pinned layer 20 and the magnetization in the free layer 18 is changed in response to the magnetic signal on the magnetic disk 2.
FIGS. 5A and 5B show the two operational states of the spin-valve magnetic sensor 22, wherein FIG. 5A shows the case in which the magnetization Q in the pinned layer 20 is anti-parallel with respect to the magnetization S in the free layer 18, while FIG. 5B shows the case in which the magnetization Q is parallel to the magnetization T in the free layer 18.
In the state of FIG. 5A, the electrons in the pinned layer 20 are polarized either to an up-spin state or down-spin state, while the electrons in the free layer 18 are polarized to an opposite spin state. Thereby, there occurs no substantial flow of the electrons from the pinned layer 20 to the free layer 18 across the non-magnetic layer 19 or vice versa, and the electrons experience a scattering. In correspondence to this, the spin-valve magnetic sensor 22 shows a high resistance.
In the state of FIG. 5B, on the other hand, the electrons in the pinned layer 20 and the electrons in the free layer 18 are polarized to the same up-spin state or down-spin state. Thereby, the electrons can flow from the pinned layer 20 to the free layer 18 or vice versa substantially freely and the spin-valve magnetic sensor 22 shows a low resistance.
FIG. 6 shows the resistance of the spin-valve magnetic sensor 22 as a function of the angle formed between the magnetization of the free layer 18 and the pinned layer 20.
As represented in FIG. 6 by a continuous line, the spin-valve magnetic sensor 22 shows a minimum resistance when the magnetization in the free layer 18 and the magnetization in the pinned layer 20 are parallel and a maximum resistance when the magnetization in the free layer 18 and the magnetization in the pinned layer are anti-parallel.
In such a spin-valve magnetic sensor, it is thus essential for the proper operation of the magnetic sensor that the magnetization of the pinned layer 20 is pinned stably. When there occurs a reversal of magnetization in the pinned layer, the output of the magnetic sensor indicative of the resistance of the spin-valve magnetic sensor would be reversed as represented in FIG. 6 by a broken line. When such a reversal is caused, it is therefore no longer possible to restore the information signal from the output of the magnetic sensor 22.
In view of the demand of miniaturization of the magnetic head used in recent magnetic storage devices for high-density recording, there is a demand also for a spin-valve magnetic sensor used therein to decrease the size of the magnetic layers for reducing the inertia of the magnetic head. On the other hand, such a decrease of size of the magnetic layers raises problem in that the effect of the surface magnetization that tends to occur so as to cancel out the magnetization of the magnetic layer is increased. Thereby, the pinned layer easily undergoes unwanted reversal of magnetization and the operation of the spin-valve magnetic sensor becomes inevitably unstable. Such a reversal may be caused by a minute electric current such as an electrostatic discharge.
A similar problem occurs also in a TMR (tunneling-magneto-resistance) magnetic sensor that uses a pinned layer.
Accordingly, it is a general object of the present invention to provide a novel and useful magnetic information storage device having a high-sensitivity magnetic sensor and a signal processing circuit for processing an output of the high-sensitivity magnetic sensor, wherein the foregoing problems are eliminated.
Another object of the present invention is to provide a signal processing circuit for processing an output of a magnetic sensor such as the one used in a magnetic information storage device wherein reproducing of information signal is achieved properly based on the output of the magnetic sensor even in such a case there occurs a reversal of magnetization in the magnetic layer constituting the magnetic sensor.
Another object of the present invention is to provide a signal processing circuit of a magnetic sensor such as the one for use in a magnetic information storage device, comprising:
a polarity detection unit supplied with an output signal of the magnetic sensor, said polarity detection unit detecting a polarity of said output signal; and
a polarity control unit supplied with said output signal of said magnetic sensor, said polarity control unit producing an output signal corresponding to said output signal of said magnetic sensor with a controlled polarity controlled in response to a result of detection of said polarity detection unit.
According to the present invention, it is possible to operate a magnetic information storage device stably even in such a case there occurs a reversal of magnetization in the magnetic layer constituting the magnetic sensor in the magnetic head.
Other objects and further features of the present invention will become apparent from the following detailed description when read in conjunction with the attached drawings.